This invention relates in general to magnetic bearings and electromagnetic motors. More specifically, it relates to a process and apparatus providing an integrated, highly compact, integrated rotary magnetic bearing and motor particularly one useful in semiconductor wafer processing.
In semiconductor fabrication, fragile wafers of silicon or other semiconducting materials must be processed in a controlled, super-clean atmosphere, whether that atmosphere is a vacuum, an inert gas, or a process gas. Microscopic contaminates in the atmosphere are a severe problem since they can deposit on the wafer, either directly or as a result of a gaseous processing of the wafer. Microscopic particles on a wafer contaminate it; semiconductor devices made from the contaminated portion of the wafer will be defective. Cleanliness is therefore directly related to yield, which in turn affects final product cost. Modern processing techniques include using multiple chambers connected by vacuum or inert gas-filled transports. These chambers are specialized for certain process steps. Processing usually involves hostile atmospheres of highly corrosive gases and high temperatures.
One processing step is an annealing of the wafer following doping by ion implantation. The implantation produces strains in the crystal structure which, if not relieved, cause unwanted variations in the resistivity of the ion doped silicon. The annealing is important to relieve these stresses. Modern fabrication has turned to rapid thermal processing (RTP) for annealing. RTP involves the use of a radiant heat source (e.g., a lamp) mounted in a vacuum chamber over the wafer. The lamp quickly brings the wafer to a suitably high temperature for the processing. For RTP, it is necessary to have the region over and under the wafer unobstructed.
Uniformity of processing over the entire wafer is important because the wafer is eventually subdivided into multiple discrete devices, e.g., microprocessor or memory chips, that each must have generally the same known characteristics. Uniformity is also important where it is desired to use a relatively large surface area for a single device, e.g., fabrication of an entire computer on a single chip. In short, uniformity also influences yield. To produce uniformity, it is conventional to rotate the wafer about a vertical or z-axis perpendicular to, and centered on, the wafer as it is being processed. Rotation is also used for other wafer processing such as chemical vapor deposition, heat treatment, doping by ion implantation, and doping by other techniques.
Heretofore conventional and RTP wafer processing equipment has used mechanical contact bearings to support a rotatable platform which in turn supports a wafer carrier. Mechanical contact bearings, however, even the custom designed, highly expensive bearings now in use, are the source of many problems. First, because they make moving contact, they wear. This wear is the source of particle contamination. Second, due to the wear and operation in a difficult environment (an ultrahigh vacuum) or hostile environment, (e.g., a corrosive or high temperature atmosphere), there are severe constraints on lubrication, the bearings fail unpredictably, and they typically fail in a comparatively short time. (Lubricants evaporate particularly when exposed to a high vacuum. Seals and choice of lubricants can help to control the problem, but bearing lubrication remains a source of contaminants.)
Bearing failure is the cause of significant production down-time. It also raises manufacturing costs due to the direct cost of new bearings and often the loss of a wafer being processed, which may itself have a value in the range of $50,000. Even without bearing failure, wafers break. Besides the loss of the wafer, breakage also produces a significant loss of production time since any wafer break produces wafer fragments and particles which must be meticulously cleaned out of the bearings before the chamber can be used again. When operating, mechanical bearings also produce vibration (they are noisy) and they transmit, and can be worn or damaged by, external mechanical shock and vibration. Mechanical bearings also suffer from stiction and "play" which introduce error in position control which adversely affect quality and yield. These and other problems also impose a practical upper limit on the size of the wafers that are processed and the speed of rotation of the wafer. At present wafers with diameter of about 200 mm are the largest which can be processed reliably. Rotational speeds are typically no greater than 90 rpm for these large wafers.
Magnetic handling has been considered for use in wafer processing. For example, M. Ota et al. in "Development Of Mag-Lev Polar Coordinate Robot Operation in Ultra High Vacuum", Magnetic Suspension II, pp. 351-359 (1991) describe a polar coordinate robot for operation in a vacuum using magnetic bearings. Magnetic bearings for contactless suspension and linear movement of wafers is also the subject of papers such as S. Moriyama et al., "Development of Magnetically Suspended Linear Pulse Motor for Contactless Direct Drive in Vacuum Chamber Robot", Kyushu Institute of Technology, Transaction of the Institute of Electrical Engineers of Japan, Vol. 115-D, No. 3, March 1995, pp. 311-318. While the advantages of magnetic suspensions to avoid of contamination problems of mechanical bearings are clear cut, this kind of research has been limited to wafer transport, particularly linear wafer transport between process chambers, not for use in a chamber itself to rotate the wafer during processing.
The general use of magnetic bearings to support a rotary motion are known. The are typically used where it is important to eliminate frictional contact totally. In a common arrangement, a pair of axially spaced magnetic bearings support a rotary shaft which is driven by an axially interposed electric motor. The rotor of the motor is typically secured to the shaft, which is the output shaft of the motor. Magnetic bearings, both radial and axial, are also used in devices as diverse as gyroscopes, flywheels, gas turbines, and electrical measuring instruments.
While the advantage of frictionless operation and operation at a distance to control movement in a sealed chamber or the like are clear, magnetic bearings are not widely used because they are bulky and expensive. Cost is driven up in part by the need for position sensors and active feedback control circuitry to suspend and center the bearing in a preselected spatial location and orientation, typically involving control over six degrees of freedom--linear motion along three mutually orthogonal (x,y, and z) axes and rotation about each of these axes.
It is also known to reduce the cost of active control by passive control of at least one degree of freedom. In a common form, this passive control uses magnetic repulsion between a pair of permanent magnets arrayed along one axis. LC tuned circuits are also known to vary a magnetic field generating current in a coil of inductance value L in a manner which provides a passive control. The coil inductance L and a capacitor with a value C are series coupled. An A.C. excitation frequency is set just above the LC resonant frequency. Because the inductance of the coil is very sensitive to the air gap between the coil and the rotor, changes in the gap automatically produce changes in the impedance of the LC circuit, which in turn adjusts the current flow to induce a centering.
Regardless of whether passive controls are used, if a driven rotating member is magnetically supported, it must also be driven. With a direct mechanical drive, there is the problem of how to transmit the rotary power into a chamber without a frictional contact that is open to the chamber (a source of contaminants) while maintaining a controlled atmosphere within the chamber (e.g. a rotating shaft held in a seal). A magnetic drive can overcome this problem, but it introduces the bulk and cost of this type of drive, as well as further problems such as the interaction of an AC flux of the drive with a DC flux of the suspension. For sealed chamber processing, the AC drive flux produces further complications. The AC flux produces eddy current losses in the chamber wall as well as losses in the stator and rotor. Second, the air gap that accommodates the chamber wall constitutes a significant source of reluctance in the magnetic circuit. Third, the AC flux acting on the rotor competes with the substantial saturation of the rotor by the DC suspension flux. In short, there are significant design consideration that lead away from integrating a rotary magnetic drive with a magnetic suspension.
To date, no known system uses the frictionless suspension of magnetic bearings in wafer processing, such as RTP in combination with a rotary electromagnetic drive of the wafer. More generally, no compact, cost-effective arrangement has been devised which integrates the levitation and frictionless operation of a magnetic bearing with a rotary electromagnetic motor drive.
It is therefore a principal object of this invention to provide a process and apparatus for a combined, integral rotary magnetic bearing and rotary drive that uses no physical contact between moving parts during its operation.
A further principal object of this invention is to provide such an integrated magnetic bearing and drive process and apparatus which are highly compact, both physically and in the active control needed to establish and maintain the bearing gap.
Another object is to provide an integrated magnetic bearing and drive with the foregoing advantages which can operate reliably and with a long life in a vacuum, in a corrosive atmosphere, or at high temperatures such as those encountered in wafer processing, particularly RTP processing.
A further object is to provide an integrated magnetic bearing and drive with the foregoing advantages which can process very large diameter (e.g., &gt;300 mm diameter) wafers and rotate them at over speed ranges from 50 to 1200 rpm.
A still further object is to provide a magnetic bearing and drive for wafer processing which reduces the particulate contaminants in the chamber and production down-time due to particle contamination or mechanical bearing failure.
Another object is to provide an integrated magnetic bearing and drive which allows a high degree of precision in positioning, produces no vibration, and isolates the wafer from external vibration and shock.
Still another object is to provide these advantages with a competitive cost of manufacture.